There is an image forming apparatus that forms an image on a sheet while transporting a sheet-like recording medium (hereinafter, collectively referred to as “sheet”) such as a sheet. The image forming apparatus includes an apparatus having an image forming unit that performs a transfer step in one step and an apparatus having an image forming unit that performs a transfer step in two steps of primary transfer and secondary transfer. In the image forming unit that performs the transfer step in two steps, a visible image (toner image) which is developed by a developing device on a photosensitive member is primarily transferred on a surface of an intermediate transfer belt which is in contact with the photosensitive member. The toner image that is primarily transferred on the surface of the intermediate transfer belt is transferred to the sheet by applying a transfer bias having the same polarity as toner to the intermediate transfer belt. The toner image that is secondarily transferred to the sheet is heated and pressed by a predetermined fixing temperature, and is fixed on the sheet in a fixing unit.
There is also toner (decolorable toner) having a decoloring function capable of eliminating a color of toner by applying a predetermined temperature (decoloring temperature). The toner is decolorized by applying the decoloring temperature to a sheet on which an image is formed with the decolorable toner and the sheet can be reused. For decoloring of the toner on the sheet, it is possible to use a device dedicated to decoloring or it is possible to use an image forming apparatus. In order to decolorize the decolorable toner in the image forming apparatus, a temperature of the fixing device is set to the decoloring temperature higher than a temperature used for an ordinary image forming operation and decoloring of the sheet passing through the fixing unit is performed. In the image forming apparatus, since a transport route of the sheet to a fixing device is the same as that of the ordinary image forming operation, the sheet passes through the transfer unit even in a decoloring operation. That is, the image forming apparatus causes an intermediate belt to be rotated to transport the sheet even in the decoloring operation.
However, when the intermediate belt is rotated, the photosensitive member, which is in contact with the intermediate belt, and an auger for agitating a developer or the like in a developing unit, or the like are also rotated. In the developing unit, when the augur or the like is rotated without supplying the toner to the photosensitive member, agitation proceeds in a state where the toner is not used for development. Therefore, during the decoloring operation, the toner of the developing unit is deteriorated due to separation of a toner surface or the like due to agitation and a charge amount of the toner is lowered. When the charge amount of the toner decreases, the toner adheres to the intermediate transfer belt and the toner deposits on the secondary transfer roller or the like. Therefore, a so-called “fogging” is likely to occur on a sheet, particularly, on a back surface of the sheet. In Particular, if both the decolorable toner and a non-decolorable toner are contained in the image forming apparatus, if fogging occurs with the non-decolorable toner other than the decolorable toner in the decoloring operation, whenever the sheet passes through the transfer unit, dirt of the non-decolorable toner adheres to the sheet and the number of times of reusing of the sheet may decrease.